1. Field of the Invention
This invention relates to a driving apparatus for an optical modulator of the electrical field absorption type for use with an optical communication system or an optical information processing system and a driving apparatus for a modulator integrated light source wherein the optical modulator is integrated together with a light source, and more particularly to a driving apparatus for an optical modulator and a driving apparatus for a modulator integrated light source by which a good eye pattern is obtained with modulated output light.
2. Description of the Related Art
In recent years, as the speed and the distance of communication of an optical communication system increase, drawbacks of a direct modulation method for a semiconductor laser which has conventionally been used are becoming a reality. The direct modulation method is a technique for outputting modulated light directly from a semiconductor laser and has a problem in that so-called wavelength chirping occurs wherein the oscillation wavelength varies transiently upon modulation and deteriorates the transmission waveform in an optical fiber. This phenomenon appears more conspicuously as the transmission speed of a signal increases and as the transmission distance increases, and becomes a serious problem particularly with an optical communication system which employs an existing 1.3 .mu.m zero dispersion normal fiber. Further, if wavelength chirping occurs, even if it is tried to increase the transmission distance using a light source of the wavelength 1.55 .mu.m band with which the fiber propagation loss is comparatively low, the transmission distance is limited by a dispersion of the fiber (a phenomenon that occurs become the refractive index of the fiber is different between different wavelengths, that is, the propagation velocity is different between different wavelengths).
However, the problems which arise from wavelength chirping can be reduced by adopting an external modulation method wherein a semiconductor laser is driven to emit light with a fixed power and the light is modulated using an optical modulator separate from the semiconductor laser.
While known optical modulators used in the external modulation method employ a dielectric such as LiNbO.sub.3 or a semiconductor such as InP or G&aAs, expectations are increasing for optical semiconductors made of a semiconductor which can be integrated together with another optical element such as a semiconductor laser or an optical amplifier or with an electronic circuit element such as an FET and can be formed in a small size and operate with a low voltage.
Available as optical modulators which employ a semiconductor are optical modulators of the electrical field absorption type (hereinafter referred to as electrical field absorption type optical modulators) which effect optical intensity modulation making use of an effect that the absorption wavelength of light is shifted to the longer wavelength side (the light absorption coefficient is varied) by an applied voltage such as a Franz-Keldysh effect of a bulk semiconductor and a quantum confined Stark effect of a multi quantum well structure and optical modulators of the Mach-Zehnder type which make use of a variation in refractive index caused by a Pockels effect or a quantum confined Stark effect of a bulk semiconductor.
With the optical modulator of the Mach-Zehnder type, wavelength chirping can be reduced to zero in principle. However, since it has a structure of the interference type and does not have a structure which has such a simple linear waveguide as in the electrical field absorption type optical modulator, it is complicated in methods of producing and driving the same.
On the other hand, while the electrical field absorption type optical modulator provides a much smaller amount of wavelength chirping than that of the direct modulation method of a semiconductor laser, it cannot still reduce the wavelength chirping to zero. However, a pre-bias application method has been proposed recently wherein a fixed voltage is applied to the electrical field absorption type optical modulator and a signal is superposed on the fixed voltage. It has been confirmed that the pre-bias application method can reduce wavelength chirping of an optical modulator to raise the dispersion withstanding property of an optical fiber with respect to laser light of the 1.55 .mu.m band and overcome the limitation to the transmission distance by the dispersion.
As an example of the type described, a device wherein a DFB (distributed feedback) laser and the electrical field absorption type optical modulator are integrated is reported in Morito et al., Collection of Lecture Papers of the Meeting in 1995 of the Electronics Society of the Electronic Information Communication Society of Japan, Separate Volume 1, p.301 (Lecture Number: C-301).
In this report, a fixed pre-bias (-1.1 V) is applied in advance to the electrical field absorption type optical modulator to increase the dispersion withstanding property of an optical fiber to achieve data transmission of 100 km at 10 Gb/s. In this manner, while the electrical field absorption type optical modulator has a structure which includes a simple linear waveguide, also in regard to reduction of the wavelength chirping, it has been or is being improved to such a degree that it has a performance higher than the other optical wavelengths.
However, in such conventional electrical field absorption type optical modulators as described above, since the relationship of the output power (optical output) to the applied voltage is not linear as seen from FIG. 1, a cross point of an eye pattern of modulated output light outputted from the optical modulator is displaced to the lower side (OFF side of light) from the center as seen in FIG. 2, and there is a problem in that, because of the displacement of a cross point, an eye mask specification required for an optical communication system is not satisfied.
More particularly, since the relationship (hereinafter referred to as quenching characteristics) of the absorbed light amount with respect to the applied voltage of the electrical field absorption type optical modulator presents a smooth curve which exhibits a comparatively linear increase with a low applied voltage and soon exhibits a gradual saturation, a cross point of an eye pattern is displaced to the lower side (OFF side of light) from the center by the non-linear quenching characteristics.
In order to solve this problem, it is necessary to either devise the structure of the electrical field absorption type optical modulator to change the non-linear quenching characteristics to a linear characteristics or devise the driving apparatus to convert the non-linear quenching characteristics into an apparently linear quenching characteristics.